It is now apparent that in eucaryots mRNA population can be subdivided into classes based on the regulation of their ultimate expression as protein. That is, not all mRNA is translated immediately after its synthesis. There is also evidence that the selective usage of mRNA subsets is an aspect of developmental regulation. Naturally occuring regulator molecules that either bring about or prevent tissue development and differentiation appear in some cases to act post-transcriptionally by selectively regulating mRNA translation. The embryogenesis and germination of the cotton seed affords an ideal system for studying this aspect of eucaryot developmental regulation. Some of the mRNAs to be used in driving seed germination are synthesized during seed embryogenesis, but their translation is prevented in vivo during embryogenesis by the plant hormone Abscisic Acid (ABA). ABA does not prevent the translation of embryogenic mRNA, and by preventing the translation of the germination mRNA prevents seed vivipary. Dissected, immature embryos, removed from the source of ABA, germinate precociously. Thus populations of seed mRNA can be subdivided into at least 2 classes based on the effect of ABA on their translation. We would like to study the mechanisms(s) by which ABA effects this selective prohibition of mRNA use and to elucidate how the two classes of mRNA differ. We plan to catalogue the principal proteins synthesized in ABA repressed and ABA-free immature embryos germinating precociously. Messenger RNAs for proteins representative of the 2 classes of mRNA will be isolated, and the pre-mRNA sequences precursor to the mRNAs purified through their hybridization with cDNA. Chemical and organizational features of the pre-mRNAs representative of the two mRNA subsets will be measured. The effect of ABA in conjunction with protein fractions from ABA-repressed and ABA-free embryos on the translation of the purified mRNAs and of mRNA from the two groups of embryos will be determined. In this manner we hope to discern the structural differences between mRNA subsets that constitute the molecular basis for the ABA repression of development.